1. Field of the Invention
The present invention relates to a method of making a capacitor element to be incorporated in a solid electrolytic capacitor which is produced with the use of a valve metal such as tantalum or niobium.
2. Description of the Related Art
A conventional capacitor element of the above-mentioned type is made in the following manner, for example.
First, powder of a valve metal, e.g., tantalum, is compressed and baked to produce an anode chip. Then, the anode chip is dipped into a chemical solution (e.g. an aqueous solution of phosphoric acid) for anodic oxidation. Thus, a highly insulating dielectric layer of tantalum pentoxide (Ta2O5) is formed on the metal particles of the anode chip. Then, a solid electrolyte layer of manganese dioxide (MnO2) is formed on the Ta2O5-dielectric layer by dipping the anode chip into an aqueous solution of manganese nitrate and then baking the chip at 200–250° C. after taken out of the solution (this dipping and baking process may be repeatedly performed a number of times). Then, a graphite layer is formed on the MnO2-electrolyte layer by dipping the anode chip into a graphite suspension and then drying the chip at 150–200° C. after taken out of the suspension (this dipping and drying process may also be performed repeatedly). Finally, a metal layer is formed on the graphite layer by application of a conductive paste. As a result, the cathode of the obtained capacitor element has a three-layer structure consisting of the solid electrolyte layer, the graphite layer and the metal layer.
In the above conventional capacitor element, the MnO2-electrolyte layer and the overlapping graphite layer greatly differ in physical properties, in particular, have considerably different thermal expansion coefficients. Therefore, when heated (this may happen when the capacitor is soldered to a printed circuit board, for example), the electrolyte layer and the graphite layer tend to expand at different rates, thereby causing delamination therebetween. Unfavorably, this leads to the deterioration of the impedance characteristics of the capacitor device.
A remedy for the above problem is disclosed in Japanese patent application laid-open No. 7(1995)-22288, for example. Specifically, this prior art document teaches that the above-mentioned delamination can be prevented by forming an additional solid electrolyte layer between the MnO2-electrolyte layer and the graphite layer with the use of a manganese dioxide material which contains 5–50 wt % of graphite powder.
According to the JP document, the intermediate graphite-containing electrolyte layer is formed in the following manner. First, a graphite-containing solution is prepared by adding an appropriate amount of graphite powder into an aqueous solution of manganese nitrate. Then, an anode chip (with the underlying MnO2-electrolyte layer formed previously) is dipped into the graphite-containing manganese nitrate solution. Finally, the anode chip is taken out of the solution and baked.
In accordance with the teaching of the prior art document, the additional solid electrolyte layer should contain 5 wt % at least. To this end, 70 wt % of manganese nitrate solution and 30 wt % of graphite powder need to be mixed, as disclosed in the description of the embodiments.
The inventor of the present application conducted experiments and found that there is a drawback in the prior art method. As noted above, a large amount of graphite power (30 wt %) is added to the manganese nitrate solution for making the additional electrolyte layer, which contains at least 5 wt % of graphite powder. With such a high mixing rate, however, the resultant electrolyte layer tends to have a considerably rough surface to an extent that the irregularity cannot be satisfactorily smoothed out merely by overlapping the upper layers (i.e., the graphite layer and the metal layer). Accordingly, the obtained capacitor element does not look fine in appearance.